Fusion formable sodium free glass

ABSTRACT

A compositional range of fusion-formable, high strain point sodium free, silicate, aluminosilicate and boroaluminosilicate glasses are described herein. The glasses can be used as substrates for photovoltaic devices, for example, thin film photovoltaic devices such as CIGS photovoltaic devices. These glasses can be characterized as having strain points≧540° C., thermal expansion coefficient of from 6.5 to 10.5 ppm/° C., as well as liquidus viscosities in excess of 50,000 poise. As such they are ideally suited for being formed into sheet by the fusion process.

This application claims the benefit of priority to U.S. ProvisionalApplication No. 61/182,404 filed on May 29, 2009.

BACKGROUND

1. Field

Embodiments relate generally to sodium free glasses and moreparticularly to fusion formable sodium free glasses which may be usefulin photochromic, electrochromic, Organic Light Emitting Diode (OLED)lighting, or photovoltaic applications, for example, thin filmphotovoltaics.

2. Technical Background

The fusion forming process typically produces flat glass with optimalsurface and geometric characteristics useful for many electronicsapplications, for instance, substrates used in electronics applications,for example, display glass for LCD televisions.

Over the last 10 years, Corning fusion glass products include 1737F™,1737G™, Eagle2000F™, EagleXG™, Jade™, and Codes 1317 and 2317 (GorillaGlass™). Efficient melting is generally believed to occur at atemperature corresponding to a melt viscosity of about 200 poise (p).These glasses share in common 200 p temperatures in excess of 1600° C.,which can translate to accelerated tank and electrode corrosion, greaterchallenges for fining due to still more elevated finer temperatures,and/or reduced platinum system life time, particularly around the finer.Many have temperatures at 3000 poise in excess of about 1300° C., andsince this is a typical viscosity for an optical stirrer, the hightemperatures at this viscosity can translate to excessive stirrer wearand elevated levels of platinum defects in the body of the glass.

Many of the above described glasses have delivery temperatures in excessof 1200° C., and this can contribute to creep of isopipe refractorymaterials, particularly for large sheet sizes.

These attributes combine so as to limit flow (because of slow meltrates), to accelerate asset deterioration, to force rebuilds ontimescales much shorter than product lifetimes, to force unacceptable(arsenic), expensive (capsule) or unwieldy (vacuum fining) solutions todefect elimination, and thus contribute in significant ways to the costof manufacturing glass.

In applications in which rather thick, comparatively low-cost glass withless extreme properties is required, these glasses are not onlyoverkill, but prohibitively expensive to manufacture. This isparticularly true when the competitive materials are made by the floatprocess, a very good process for producing low cost glass with ratherconventional properties. In applications that are cost sensitive, suchas large-area photovoltaic panels and OLED lighting, this costdifferential is so large as to make the price point of LCD-type glassesunacceptable.

To reduce such costs, it is advantageous to drive down the largestoverall contributors (outside of finishing), and many of these trackdirectly with the temperatures used in the melting and forming process.Therefore, there is a need for a glass that melts at a lower temperaturethan those aforementioned glasses.

Further, it would be advantageous to have a glass useful for lowtemperature applications, for instance, photovoltaic and OLED lightapplications. Further, it would be advantageous to have a glass whoseprocessing temperatures were low enough that the manufacturing of theglass would not excessively consume the energy that these applicationsare aiming to save.

SUMMARY

A compositional range of fusion-formable, high strain point sodium free,silicate, aluminosilicate and boroaluminosilicate glasses useful, forexample, for thin-film photovoltaic applications are described herein.More specifically, these glasses are advantageous materials to be usedin copper indium gallium diselenide (CIGS) photovoltaic modules wherethe sodium required to optimize cell efficiency is not to be derivedfrom the substrate glass but instead from a separate deposited layerconsisting of a sodium containing material such as NaF. Current CIGSmodule substrates are typically made from soda-lime glass sheet that hasbeen manufactured by the float process. However, use of higher strainpoint glass substrates can enable higher temperature CIGS processing,which is expected to translate into desirable improvements in cellefficiency. Moreover, it may be that the smoother surface offusion-formed glass sheets yields additional benefits, such as improvedfilm adhesion, etc.

Accordingly, the sodium free glasses described herein can becharacterized by strain points 540° C., for example, 560° C. so as toprovide advantage with respect to soda-lime glass and/or liquidusviscosity 30,000 poise to allow manufacture via the fusion process. Inorder to avoid thermal expansion mismatch between the substrate and CIGSlayer, the inventive glasses are further characterized by a thermalexpansion coefficient in the range of from 6.5 to 10.5 ppm/° C.

One embodiment is a glass comprising, in weight percent:

-   -   35 to 75 percent SiO₂;    -   0 to 15 percent Al₂O₃;    -   0 to 20 percent B₂O₃;    -   3 to 30 percent K₂O;    -   0 to 15 percent MgO;    -   0 to 10 percent CaO;    -   0 to 12 percent SrO;    -   0 to 40 percent BaO; and    -   0 to 1 percent SnO₂,

wherein the glass is substantially free of Na₂O.

In another embodiment, the glass comprises, in weight percent:

-   -   35 to 75 percent SiO₂;    -   greater than 0 to 15 percent Al₂O₃;    -   greater than 0 to 20 percent B₂O₃;    -   3 to 30 percent K₂O;    -   greater than 0 to 15 percent MgO;    -   greater than 0 to 10 percent CaO;    -   greater than 0 to 12 percent SrO;    -   greater than 0 to 40 percent BaO; and    -   greater than 0 to 1 percent SnO₂,

wherein the glass is substantially free of Na₂O.

In another embodiment, the glass comprises, in weight percent:

-   -   39 to 75 percent SiO₂;    -   2 to 13 percent Al₂O₃;    -   1 to 11 percent B₂O₃;    -   3 to 30 percent K₂O;    -   0 to 7 percent MgO;    -   0 to 10 percent CaO;    -   0 to 12 percent SrO;    -   0 to 40 percent BaO; and    -   0 to 1 percent SnO₂,

wherein the glass is substantially free of Na₂O.

In another embodiment, the glass comprises, in weight percent:

-   -   50 to 70 percent SiO₂;    -   2 to 13 percent Al₂O₃;    -   1 to 11 percent B₂O₃;    -   3 to 30 percent K₂O;    -   0 to 7 percent MgO;    -   0 to 7 percent CaO;    -   0 to 5 percent SrO;    -   1 to 40 percent BaO; and    -   0 to 0.3 percent SnO₂,

wherein the glass is substantially free of Na₂O.

Another embodiment is a glass consisting essentially of, in weightpercent:

-   -   35 to 75 percent SiO₂;    -   0 to 15 percent Al₂O₃;    -   0 to 20 percent B₂O₃;    -   3 to 30 percent K₂O;    -   0 to 15 percent MgO;    -   0 to 10 percent CaO;    -   0 to 12 percent SrO;    -   0 to 40 percent BaO; and    -   0 to 1 percent SnO₂,

wherein the glass is substantially free of Na₂O.

Another embodiment is a glass comprising, in weight percent:

-   -   45 to 75 percent SiO₂;    -   3 to 15 percent Al₂O₃;    -   0 to 20 percent B₂O₃;    -   14 to 25 percent K₂O;    -   0 to 15 percent MgO;    -   0 to 10 percent CaO;    -   0 to 12 percent SrO;    -   0 to 40 percent BaO; and    -   0 to 1 percent SnO₂,

wherein the glass is substantially free of Na₂O and wherein the glass isfusion formable and has a strain point of 540° C. or greater, acoefficient of thermal expansion of 50×10⁻⁷ or greater, T₂₀₀ less than1630° C., and having a liquidus viscosity of 150,000 poise or greater.

Additional features and advantages will be set forth in the detaileddescription which follows, and in part will be readily apparent to thoseskilled in the art from the description or recognized by practicing theinvention as described in the written description and claims hereof.

It is to be understood that both the foregoing general description andthe following detailed description are merely exemplary of theinvention, and are intended to provide an overview or framework forunderstanding the nature and character of the invention as it isclaimed.

DETAILED DESCRIPTION

Reference will now be made in detail to various embodiments of theinvention.

As used herein, the term “substrate” can be used to describe either asubstrate or a superstrate depending on the configuration of thephotovoltaic cell. For example, the substrate is a superstrate, if whenassembled into a photovoltaic cell, it is on the light incident side ofa photovoltaic cell. The superstrate can provide protection for thephotovoltaic materials from impact and environmental degradation whileallowing transmission of the appropriate wavelengths of the solarspectrum. Further, multiple photovoltaic cells can be arranged into aphotovoltaic module. Photovoltaic device can describe either a cell, amodule, or both.

As used herein, the term “adjacent” can be defined as being in closeproximity. Adjacent structures may or may not be in physical contactwith each other. Adjacent structures can have other layers and/orstructures disposed between them.

One embodiment is a glass comprising, in weight percent:

-   -   35 to 75 percent SiO₂;    -   0 to 15 percent Al₂O₃;    -   0 to 20 percent B₂O₃;    -   3 to 30 percent K₂O;    -   0 to 15 percent MgO;    -   0 to 10 percent CaO;    -   0 to 12 percent SrO;    -   0 to 40 percent BaO; and    -   0 to 1 percent SnO₂,

wherein the glass is substantially free of Na₂O.

Another embodiment is a glass comprising, in weight percent:

-   -   45 to 75 percent SiO₂;    -   3 to 15 percent Al₂O₃;    -   0 to 20 percent B₂O₃;    -   14 to 25 percent K₂O;    -   0 to 15 percent MgO;    -   0 to 10 percent CaO;    -   0 to 12 percent SrO;    -   0 to 40 percent BaO; and    -   0 to 1 percent SnO₂,

wherein the glass is substantially free of Na₂O and wherein the glass isfusion formable and has a strain point of 540° C. or greater, acoefficient of thermal expansion of 50×10⁻⁷ or greater, T₂₀₀ less than1630° C., and having a liquidus viscosity of 150,000 poise or greater.

In another embodiment, the glass consists essentially of in weightpercent:

-   -   45 to 75 percent SiO₂;    -   3 to 15 percent Al₂O₃;    -   0 to 20 percent B₂O₃;    -   14 to 25 percent K₂O;    -   0 to 15 percent MgO;    -   0 to 10 percent CaO;    -   0 to 12 percent SrO;    -   0 to 40 percent BaO; and    -   0 to 1 percent SnO₂,

wherein the glass is substantially free of Na₂O and wherein the glass isfusion formable and has a strain point of 540° C. or greater, acoefficient of thermal expansion of 50×10⁻⁷ or greater, T₂₀₀ less than1630° C., and having a liquidus viscosity of 150,000 poise or greater.

The glass is substantially free of Na₂O, for example, the content ofNa₂O can be 0.05 weight percent or less, for example, zero weightpercent. The glass, according to some embodiments, is free ofintentionally added sodium.

In some embodiments, the glass comprises greater than 3 percent K₂O, forexample, greater than 5 percent K₂O, for example, greater than 10percent K₂O, for example, greater than 12 percent K₂O, for examplegreater than 13.5 percent K₂O, for example, greater than 15 percent K₂O.

Since the glass is substantially free of Na₂O, in some embodiments, theweight percent of the combination of Na₂O and K₂O is greater than 3percent, for example, greater than 5 percent, for example, greater than10 percent, for example, greater than 12 percent, for example greaterthan 13.5 percent, for example, greater than 15 percent.

In some embodiments, the glass comprises at least 45 percent SiO₂, forexample, at least 50 percent SiO₂, for example, at least 60 percentSiO₂.

The glass, in one embodiment, is rollable. The glass, in one embodiment,is down-drawable. The glass can be slot drawn or fusion drawn, forexample. According to another embodiment the glass can be float formed.

The glass can further comprise 3 weight percent or less, for example, 0to 3 weight percent, for example, greater than 0 to 3 weight percent,for example, 1 to 3 weight percent of TiO₂, MnO, ZnO, Nb₂O₅, MoO₃,Ta₂O₅, WO₃, ZrO₂, Y₂O₃, La₂O₃, HfO₂, CdO, SnO₂, Fe₂O₃, CeO₂, As₂O₃,Sb₂O₃, Cl, Br, or combinations thereof. In some embodiments, the glassis substantially free of ZrO₂. In some embodiments, the glass issubstantially free of ZnO. The glass, in one embodiment, comprises 3weight percent or less, for example, 0 to 3 weight percent, for example,greater than 0 to 3 weight percent, for example, 1 to 3 weight percentof TiO₂.

As mentioned above, the glasses, according some embodiments, comprisegreater than 0 weight percent B₂O₃, for example, 1 weight percent ormore, or, for example, 0 to 11 weight percent, for example, greater than0 to 11 weight percent B₂O₃, for example, 0.5 to 11 weight percent B₂O₃,for example 1 to 11 weight percent B₂O₃. B₂O₃ is added to the glass toreduce melting temperature, to decrease liquidus temperature, toincrease liquidus viscosity, and to improve mechanical durabilityrelative to a glass containing no B₂O₃.

According to some embodiments, the glass is substantially free of B₂O₃.In some embodiments, the glass is substantially free of B₂O₃ andcomprises at least 45 percent SiO₂, for example, at least 50 percentSiO₂, for example, at least 60 percent SiO₂.

The glass, according to some embodiments, comprises 30 weight percenttotal RO or less wherein RO is R is an alkaline earth metal selectedfrom Mg, Ca, Ba, and Sr, for example, 20 weight percent total RO orless, for example, 15 weight percent total RO or less, for example, 13.5weight percent total RO or less.

The glass can comprise, for example, 0 to 15, greater than 0 to 15weight percent, for example, 1 to 15 weight percent MgO. The glass cancomprise, for example, 0 to 7, greater than 0 to 7 weight percent, forexample, 1 to 7 weight percent MgO. MgO can be added to the glass toreduce melting temperature and to increase strain point. It candisadvantageously lower CTE relative to other alkaline earths (e.g.,CaO, SrO, BaO), and so other adjustments may be made to keep the CTEwithin the desired range. Examples of suitable adjustments includeincrease SrO at the expense of CaO, increasing alkali oxideconcentration, and replacing a smaller alkali oxide in part with alarger alkali oxide.

According to another embodiment, the glass is substantially free of BaO.For example, the content of BaO can be 0.05 weight percent or less, forexample, zero weight percent.

In some embodiments, the glass is substantially free of Sb₂O₃, As₂O₃, orcombinations thereof, for example, the glass comprises 0.05 weightpercent or less of Sb₂O₃ or As₂O₃ or a combination thereof. For example,the glass can comprise zero weight percent of Sb₂O₃ or As₂O₃ or acombination thereof.

The glasses, in some embodiments, comprise 0 to 10 weight percent CaO,for example, 0 to 7 weight percent CaO, or, for example, greater than 0,for example, 1 to 10 weight percent CaO, for example, 1 to 7 weightpercent CaO. Relative to alkali oxides or SrO, CaO contributes to higherstrain point, lower density, and lower melting temperature.

The glasses can comprise, in some embodiments, 0 to 12 weight percentSrO, for example, greater than zero to 12 weight percent, for example, 1to 12 weight percent SrO, or for example, 0 to 5 weight percent SrO, forexample, greater than zero to 5 weight percent, for example, 1 to 5weight percent SrO. In certain embodiments, the glass contains nodeliberately batched SrO, though it may of course be present as acontaminant in other batch materials. SrO contributes to highercoefficient of thermal expansion, and the relative proportion of SrO andCaO can be manipulated to improve liquidus temperature, and thusliquidus viscosity. SrO is not as effective as CaO or MgO for improvingstrain point, and replacing either of these with SrO tends to cause themelting temperature to increase.

Alkali cations such as K raise the CTE steeply, but also lower thestrain point and, depending upon how they are added, increase meltingtemperatures. The least effective alkali oxide for CTE is Li₂O, and themost effective alkali oxide is Cs₂O.

Another embodiment is a glass consisting essentially of, in weightpercent:

-   -   35 to 75 percent SiO₂;    -   0 to 15 percent Al₂O₃;    -   0 to 20 percent B₂O₃;    -   3 to 30 percent K₂O;    -   0 to 15 percent MgO;    -   0 to 10 percent CaO;    -   0 to 12 percent SrO;    -   0 to 40 percent BaO; and    -   0 to 1 percent SnO₂,

wherein the glass is substantially free of Na₂O.

The glass, according to some embodiments, is down-drawable; that is, theglass is capable of being formed into sheets using down-draw methodssuch as, but not limited to, fusion draw and slot draw methods that areknown to those skilled in the glass fabrication arts. Such down-drawprocesses are used in the large-scale manufacture of ion-exchangeableflat glass.

The fusion draw process uses a drawing tank that has a channel foraccepting molten glass raw material. The channel has weirs that are openat the top along the length of the channel on both sides of the channel.When the channel fills with molten material, the molten glass overflowsthe weirs. Due to gravity, the molten glass flows down the outsidesurfaces of the drawing tank. These outside surfaces extend down andinwardly so that they join at an edge below the drawing tank. The twoflowing glass surfaces join at this edge to fuse and form a singleflowing sheet. The fusion draw method offers the advantage that, sincethe two glass films flowing over the channel fuse together, neitheroutside surface of the resulting glass sheet comes in contact with anypart of the apparatus. Thus, the surface properties are not affected bysuch contact.

The slot draw method is distinct from the fusion draw method. Here themolten raw material glass is provided to a drawing tank. The bottom ofthe drawing tank has an open slot with a nozzle that extends the lengthof the slot. The molten glass flows through the slot/nozzle and is drawndownward as a continuous sheet therethrough and into an annealingregion.

Compared to the fusion draw process, the slot draw process provides athinner sheet, as only a single sheet is drawn through the slot, ratherthan two sheets being fused together, as in the fusion down-drawprocess.

In order to be compatible with down-draw processes, the glass describedherein has a high liquidus viscosity. In one embodiment, the glass has aliquidus viscosity of 50,000 poise or greater, for example, 150,000poise or greater, for example, 200,000 poise or greater, for example,250,000 poise or greater, for example, 300,000 poise or greater, forexample, 350,000 poise or greater, for example, 400,000 poise orgreater, for example, greater than or equal to 500,000 poise. Theliquidus viscosities of some exemplary glasses are closely correlatedwith the difference between the liquidus temperature and the softeningpoint. For downdraw processes, some exemplary glasses advantageouslyhave liquidus-softening point less than about 230° C., for example, lessthan 200° C.

Accordingly, in one embodiment, the glass has a strain point of 540° C.or greater, for example, 550° C. or greater, for example, 560° C. orgreater, or for example, from 540° C. to 650° C. In some embodiments,the glass has a coefficient of thermal expansion of 50×10⁻⁷ or greater,for example, 60×10⁻⁷ or greater, for example, 70×10⁻⁷ or greater, forexample, 80×10⁻⁷ or greater. In one embodiment, the glass has a strainpoint of from 50×10⁻⁷ to 90×10⁻⁷.

In one embodiment, the glass has a coefficient of thermal expansion of50×10⁻⁷ or greater and a strain point of 540° C. or greater. In oneembodiment, the glass has a coefficient of thermal expansion of 50×10⁻⁷or greater and a strain point of 560° C. or greater.

According to one embodiment, the glass is ion exchanged in a salt bathcomprising one or more salts of alkali ions. The glass can be ionexchanged to change its mechanical properties. For example, smalleralkali ions, such as lithium can be ion-exchanged in a molten saltcontaining one or more larger alkali ions, such as potassium, rubidiumor cesium. If performed at a temperature well below the strain point forsufficient time, a diffusion profile will form in which the largeralkali moves into the glass surface from the salt bath, and the smallerion is moved from the interior of the glass into the salt bath. When thesample is removed, the surface will go under compression, producingenhanced toughness against damage. Such toughness may be desirable ininstances where the glass will be exposed to adverse environmentalconditions, such as photovoltaic grids exposed to hail. A large alkalialready in the glass can also be exchanged for a smaller alkali in asalt bath. If this is performed at temperatures close to the strainpoint, and if the glass is removed and its surface rapidly reheated tohigh temperature and rapidly cooled, the surface of the glass will showconsiderable compressive stress introduced by thermal tempering. Thiswill also provide protection against adverse environmental conditions.It will be clear to one skilled in the art that any monovalent cationcan be exchanged for alkalis already in the glass, including copper,silver, thallium, etc., and these also provide attributes of potentialvalue to end uses, such as introducing color for lighting or a layer ofelevated refractive index for light trapping.

According to another embodiment, the glass can be float formed as knownin the art of float forming glass.

In one embodiment, the glass is in the form of a sheet. The glass in theform of a sheet can be thermally tempered.

In one embodiment, an Organic Light Emitting Diode device comprises theglass in the form of a sheet.

The glass, according to one embodiment, is transparent.

In one embodiment, a photovoltaic device comprises the glass in the formof a sheet. The photovoltaic device can comprise more than one of theglass sheets, for example, as a substrate and/or as a superstrate. Inone embodiment, the photovoltaic device comprises the glass sheet as asubstrate and/or superstrate, a conductive material adjacent to thesubstrate, and an active photovoltaic medium adjacent to the conductivematerial. In one embodiment, the active photovoltaic medium comprises aCIGS layer. In one embodiment, the active photovoltaic medium comprisesa cadmium telluride (CdTe) layer. In one embodiment, the photovoltaicdevice comprises a functional layer comprising copper indium galliumdiselenide or cadmium telluride. In one embodiment, the photovoltaicdevice the functional layer is copper indium gallium diselenide. In oneembodiment, the functional layer is cadmium telluride.

The photovoltaic device, according to one embodiment, further comprisesa barrier layer disposed between or adjacent to the superstrate orsubstrate and the functional layer. In one embodiment, the photovoltaicdevice further comprises a barrier layer disposed between or adjacent tothe superstrate or substrate and a transparent conductive oxide (TCO)layer, wherein the TCO layer is disposed between or adjacent to thefunctional layer and the barrier layer. A TCO may be present in aphotovoltaic device comprising a CdTe functional layer. In oneembodiment, the barrier layer is disposed directly on the glass.

In one embodiment, the glass sheet is transparent. In one embodiment,the glass sheet as the substrate and/or superstrate is transparent.

According to some embodiments, the glass sheet has a thickness of 4.0 mmor less, for example, 3.5 mm or less, for example, 3.2 mm or less, forexample, 3.0 mm or less, for example, 2.5 mm or less, for example, 2.0mm or less, for example, 1.9 mm or less, for example, 1.8 mm or less,for example, 1.5 mm or less, for example, 1.1 mm or less, for example,0.5 mm to 2.0 mm, for example, 0.5 mm to 1.1 mm, for example, 0.7 mm to1.1 mm. Although these are exemplary thicknesses, the glass sheet canhave a thickness of any numerical value including decimal places in therange of from 0.1 mm up to and including 4.0 mm.

In one embodiment, an electrochromic device comprises the glass in theform of a sheet. The electrochromic device can be, for example, anelectrochromic window. In one embodiment, the electrochromic windowcomprises one or more of the glass sheets, such as in a single, double,or triple pane window.

The fusion-formable glasses of this invention, by virtue of theirrelatively high strain point, represent advantaged substrate materialsfor CIGS photovoltaic modules as they can enable higher temperatureprocessing of the critical semiconductor layers. When manufactured bythe fusion process, their superior surface quality relative to that offloat glass may also result in further improvements to the photovoltaicmodule making process. Advantageous embodiments of this invention arecharacterized by liquidus viscosity in excess of 400,000 poise, therebyenabling the fabrication of the relatively thick glass sheet that may berequired by some module manufacturers. Finally, the most advantageousembodiments of this invention comprise glasses for which the 200 poisetemperature is less than 1580° C., providing for the possibility ofsignificantly lower cost melting/forming.

EXAMPLES

The following is an example of how to fabricate a sample of an exemplaryglass, according to one embodiment of the invention, as shown inTable 1. This composition corresponds to composition number 22 shown inTable 5.

TABLE 1 oxide mol % SiO₂ 64.93 Al₂O₃ 0 MgO 17.5 CaO 0 SrO 0 B₂O₃ 0 K₂O17.5 SnO₂ 0.10 BaO 0In some embodiments, the total does not add up to 100%, since certaintramp elements are present at non-negligible concentrations.

Batch materials, as shown in Table 2 were weighed and added to a 4 literplastic container:

TABLE 2 Batch Components sand Magnesia Potassium carbonate 10% SnO₂ and90% sand

It should be appreciated that in the batch, limestone, depending on thesource can contain tramp elements and/or vary amounts of one or moreoxides, for example, MgO and/or BaO. The sand is advantageouslybeneficiated so that at least 80% by mass passes 60 mesh, for example 80mesh, for example 100 mesh. The SnO₂ added, in this example, waspre-mixed with sand at a level of 10% by weight so as to ensurehomogeneous mixing with the other components. The bottle containing thebatch materials was mounted to a tumbler and the batch materials weremixed so as to make a homogeneous batch and to break up softagglomerates. The mixed batch was transferred to a 1800 cc platinumcrucible and placed into a high-temperature ceramic backer. The platinumin its backer was loaded into a glo-bar furnace idling at a temperatureof 1600° C. After 16 hours, the crucible+backer was removed and theglass melt was poured onto a cold surface, such as a steel plate, toform a patty, and then transferred to an annealer held at a temperatureof 615° C. The glass patty was held at the annealer temperature for 2hours, then cooled at a rate of 1° C. per minute to room temperature.

Table 3, Table 4, Table 5, Table 6, Table 7, Table 8, and Table 9 showexemplary glasses, according to embodiments of the invention, and madeaccording to the above example. Properties data for some exemplaryglasses are also shown in Table 3, Table 4, Table 5, Table 6, Table 7,Table 8, and Table 9.

In the Tables T_(str)(° C.) is the strain point which is the temperaturewhen the viscosity is equal to 10^(14.7) P as measured by beam bendingor fiber elongation. T_(ann)(° C.) is the annealing point which is thetemperature when the viscosity is equal to 10^(13.18) P as measured bybeam bending or fiber elongation. T_(s)(° C.) is the softening pointwhich is the temperature when the viscosity is equal to 10^(7.6) P asmeasured by beam bending or fiber elongation. α(10⁻⁷/° C.) or a(10⁻⁷/°C.) in the Tables is the coefficient of thermal expansion (CTE) which isthe amount of dimensional change from either 0 to 300° C. or 25 to 300°C. depending on the measurement. CTE is typically measured bydilatometry. r(g/cc) is the density which is measured with theArchimedes method (ASTM C693). T₂₀₀(° C.) is the two-hundred Poise (P)temperature. This is the temperature when the viscosity of the melt is200 P as measured by HTV (high temperature viscosity) measurement whichuses concentric cylinder viscometry. T_(liq)(° C.) is the liquidustemperature. This is the temperature where the first crystal is observedin a standard gradient boat liquidus measurement (ASTM C829-81).Generally this test is 72 hours but can be as short as 24 hours toincrease throughput at the expense of accuracy (shorter tests couldunderestimate the liquidus temperature). η_(liq)(° C.) is the liquidusviscosity. This is the viscosity of the melt corresponding to theliquidus temperature.

TABLE 3 Example 1 2 3 4 5 6 7 8 9 10 Composition (mol %) K₂O 3 4.5 64.95 12.1 12.1 12 12 12 10 MgO 0 0 0 0 1.7 3.38 3.8 4.4 4 5.2 CaO 0 0 00 6.76 3.38 3.8 4.4 4 5.2 SrO 0.41 0.31 0.21 0.28 0 1.7 1.9 2.2 2 2.6BaO 20.29 15.19 10.12 13.67 0 0 0 0 0 0 B₂O₃ 18.67 14 9.33 12.6 3.033.03 3 1.5 1.5 1.5 Al₂O₃ 3 4.5 6 4.95 4.88 4.88 4 4 5 4 SiO₂ 54.67 61.568.33 63.55 71.43 71.43 71.4 71.4 71.4 71.4 SnO₂ 0 0 0 0.1 0.1 0.1 0.10.1 0.1 0.1 Composition (wt %) K₂O 3.4 5.35 7.52 5.97 17.2 17.1 17.117.1 17 14.4 MgO 0 0 0 0 1.04 2.05 2.32 2.68 2.43 3.2 CaO 0 0 0 0 5.732.84 3.21 3.73 3.37 4.46 SrO 0.51 0.41 0.29 0.37 0 2.64 2.97 3.44 3.114.12 BaO 37.3 29.4 20.7 26.9 0 0 0 0 0 0 B₂O₃ 15.6 12.3 8.64 11.2 3.183.16 3.15 1.58 1.57 1.6 Al₂O₃ 3.68 5.79 8.14 6.46 7.53 7.46 6.16 6.177.66 6.24 SiO₂ 39.4 46.6 54.6 48.9 64.9 64.3 64.7 64.9 64.5 65.6 SnO₂ 00 0 0.19 0.23 0.23 0.23 0.23 0.23 0.23 T_(str) (° C.) 578 ~580 584 588597 591 591 595 604 606 T_(ann) (° C.) 616 ~620 627 625 644 640 638 645655 657 T_(s) (° C.) 754 770 815 790 a (10⁻⁷/° C.) 71.9 70.6 65.3 66.879.9 79.6 80.2 82.5 80.5 76 r (gm/cc) 2.901 2.446 2.462 2.472 2.4832.473 2.493 T₂₀₀ (° C.) 1111 1254 1443 1410 1595 1624 1589 1617 16221613 T_(liq) (° C.) 885 905 910 910 1060 975 950 995 1085 1050 η_(liq)(kp) 37 110 724 235 129 1221 1675 910 156 298

TABLE 4 Example 11 12 13 14 15 16 17 18 19 20 Composition (mol %) K₂O 1212 12 12 10 12 12 12 12 12 MgO 3.4 3 4.2 4.8 5.6 4 4.4 5 3.67 1.84 CaO3.4 3 4.2 4.8 5.6 4 4.4 5 5.5 7.33 SrO 1.7 1.5 2.1 2.4 2.8 2 2.2 2.51.82 1.82 BaO 0 0 0 0 0 0 0 0 0.01 0.01 B₂O₃ 4 4 3 1.5 1.5 4 3 1.5 1.51.5 Al₂O₃ 4 5 3 3 3 2.5 2.5 2.5 4 4 SiO₂ 71.4 71.4 71.4 71.4 71.4 71.471.4 71.4 71.4 71.4 SnO₂ 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1Composition (wt %) K₂O 17 16.9 17.2 17.2 14.5 17.2 17.2 17.3 17.1 17 MgO2.07 1.81 2.57 2.94 3.47 2.46 2.7 3.07 2.24 1.12 CaO 2.87 2.52 3.57 4.14.83 3.41 3.76 4.28 4.67 6.2 SrO 2.65 2.33 3.3 3.78 4.46 3.15 3.47 3.952.86 2.85 BaO 0 0 0 0 0 0 0 0 0.02 0.02 B₂O₃ 4.19 4.17 3.17 1.59 1.614.23 3.18 1.6 1.58 1.58 Al₂O₃ 6.15 7.64 4.64 4.65 4.71 3.87 3.88 3.896.17 6.15 SiO₂ 64.6 64.2 65.1 65.3 66 65.3 65.4 65.6 65 64.7 SnO₂ 0.230.23 0.23 0.23 0.23 0.23 0.23 0.23 0.23 0.23 T_(str) (° C.) 585 590 585592 602 582 586 588 598 600 T_(ann) (° C.) 632 638 631 640 651 627 632636 647 648 T_(s) (° C.) a (10⁻⁷/° C.) 79.4 78.1 79.5 83.8 77 81.9 81.584.4 82 84.2 r (gm/cc) 2.466 2.451 2.483 2.494 2.502 2.485 2.489 2.4972.478 2.492 T₂₀₀ (° C.) 1596 1625 1553 1553 1556 1527 1508 1523 15851549 T_(liq) (° C.) <900 940 <950 960 1040 <850 870 940 985 1030 η_(liq)(kp) >2000 2143 >1000 1080 202 >7000 6297 1385 660 179

TABLE 5 Example 21 22 23 24 25 26 27 28 29 30 Composition (mol %) K₂O 1217.5 14.94 10.8 9.6 12.47 13.6 12.6 13.35 11.77 MgO 1.83 17.5 1.37 5.174.59 11.02 12.9 13.9 13.7 11.94 CaO 5.5 0 1.11 6.21 10.18 3.13 2.4 2.41.75 0.86 SrO 3.65 0 0.77 1.4 1.24 1.01 1.45 1.45 1.1 0.66 BaO 0.02 0 00 0 0 0 0 0 3.04 B₂O₃ 1.5 0 0.4 3.52 6.24 1.64 0.4 0.4 1.9 3.11 Al₂O₃ 40 4.17 4.32 3.84 3.13 2.35 2.35 2.3 2.76 SiO₂ 71.4 64.93 77.17 68.5164.24 67.53 66.8 66.8 65.8 65.79 SnO₂ 0.1 0.07 0.07 0.07 0.07 0.07 0.10.1 0.1 0.07 Composition (wt %) K₂O 16.8 26.3 21 15.6 13.9 18.3 20 18.719.7 16.7 MgO 1.1 11.2 0.82 3.18 2.85 6.93 8.13 8.83 8.67 7.24 CaO 4.590 0.93 5.32 8.78 2.73 2.11 2.13 1.54 0.73 SrO 5.63 0 1.19 2.22 1.98 1.642.34 2.36 1.79 1.03 BaO 0.05 0 0 0 0 0 0 0 0 7.02 B₂O₃ 1.56 0 0.42 3.746.69 1.78 0.44 0.44 2.08 3.25 Al₂O₃ 6.07 0 6.33 6.73 6.03 4.98 3.74 3.783.68 4.23 SiO₂ 63.8 62.1 69 62.9 59.4 63.3 62.8 63.3 62.1 59.5 SnO₂ 0.220.17 0.16 0.16 0.16 0.16 0.24 0.24 0.24 0.16 T_(str) (° C.) 593 610 565592 594 595 594 602 592 576 T_(ann) (° C.) 641 661 616 638 634 647 649656 641 624 T_(s) (° C.) 879 847 847 815 866 874 879 858 835 a (10⁻⁷/°C.) 84 104.1 88.6 78 77.8 85 89.9 85.7 87.9 83.7 r (gm/cc) 2.523 2.4442.418 2.483 2.512 2.472 2.483 2.485 2.468 2.558 T₂₀₀ (° C.) 1547 15351531 1502 1473 T_(liq) (° C.) 1010 <1150 <950 1050 1080 1075 1080 10801060 990 η_(liq) (kp) 251 103 109 ~75 ~86 106 290

TABLE 6 Example 31 32 33 34 35 36 37 38 39 40 Composition (mol %) K₂O 1412 11.76 11.73 10.96 16 14.61 12.19 14.1 14.1 MgO 2.9 3.9 3.82 6.57 6.140 1.38 3.79 1.33 0 CaO 2.85 3.65 3.58 5.77 5.39 5 5.32 5.89 5.14 5.13SrO 0.7 0.9 0.88 1.88 1.75 0 0 1.03 0 1.33 BaO 0 0 0 0 0 0 0 0 0 0 B₂O₃3.03 3.03 5 1.55 1.45 3 3.14 3.38 3.03 3.03 Al₂O₃ 4.9 4.9 4.8 2.91 2.725.34 5.06 4.59 4.89 4.88 SiO₂ 71.52 71.52 70.06 69.5 71.5 70.56 70.3969.06 71.49 71.42 SnO₂ 0.1 0.1 0.1 0.1 0.1 0 0.1 0.1 0.1 0.1 Composition(wt %) K₂O 19.7 17.1 16.7 17 16 22.2 20.5 17.4 19.9 19.6 MgO 1.75 2.382.33 4.07 3.83 0 0.83 2.31 0.8 0 CaO 2.39 3.09 3.02 4.98 4.67 4.13 4.454.99 4.31 4.25 SrO 1.08 1.41 1.38 2.99 2.8 0 0 1.62 0 2.04 BaO 0 0 0 0 00 0 0 0 0 B₂O₃ 3.14 3.18 5.25 1.66 1.56 3.07 3.26 3.55 3.16 3.12 Al₂O₃7.46 7.55 7.39 4.57 4.29 8 7.69 7.07 7.45 7.36 SiO₂ 64.1 64.9 63.5 64.366.4 62.4 62.8 62.7 64.2 63.4 SnO₂ 0.23 0.23 0.23 0.23 0.23 0 0.23 0.230.22 0.21 T_(str) (° C.) 597 613 608 598 607 565 588 594 595 590 T_(ann)(° C.) 645 663 658 646 658 611 633 640 640 635 T_(s) (° C.) a (10⁻⁷/°C.) 84.8 78.1 74.7 85.1 81.1 90 88 82.3 86.1 86 r (gm/cc) 2.446 2.4412.434 2.498 2.48 2.452 2.473 2.446 2.475 T₂₀₀ (° C.) 1624 1649 1648 15151557 1554 1558 1594 1567 T_(liq) (° C.) 1010 1010 960 1010 1010 10101040 1035 990 980 η_(liq) (kp) 376 886 2729 270 479 105 169 394 407

TABLE 7 Example 41 42 43 44 45 46 47 48 49 Composition (mol %) K₂O 12.3710.32 11.97 13.21 9.21 12.37 4 8 5 MgO 1.17 1.16 1.33 1.24 0 1.17 1.7512 4 CaO 4.51 4.49 2.13 4.81 5.14 4.51 7.02 1 7 SrO 0 1.18 2.99 0 4.14 02.91 7 1 BaO 0 1.18 1.69 0 2 0 3.32 7 12 B₂O₃ 2.66 2.64 2.99 2.84 3.033.36 10.67 1 0 Al₂O₃ 4.28 4.26 4.99 4.58 4.89 4.28 8.52 0 0 SiO₂ 75.0174.65 71.81 73.23 71.49 74.01 62.25 64 71 SnO₂ 0.1 0.1 0.1 0.1 0.1 0.10.1 0.1 0.1 Composition (wt %) K₂O 17.6 14.5 16.3 18.7 12.5 17.6 5.3610.7 6.5 MgO 0.71 0.7 0.77 0.75 0 0.71 1.01 6.89 2.22 CaO 3.82 3.75 1.734.05 4.16 3.82 5.6 0.8 5.41 SrO 0 1.83 4.47 0 6.2 0 4.3 10.3 1.43 BaO 02.71 3.73 0 4.43 0 7.24 15.3 25.4 B₂O₃ 2.8 2.75 3.01 2.97 3.05 3.54 10.60.99 0 Al₂O₃ 6.59 6.48 7.34 7.02 7.2 6.6 12.4 0 0 SiO₂ 68.1 66.9 62.366.1 62.1 67.3 53.2 54.7 58.8 SnO₂ 0.23 0.22 0.22 0.23 0.22 0.23 0.210.21 0.21 T_(str) (° C.) 602 604 592 599 623 603 582 608 T_(ann) (° C.)651 653 639 647 672 651 628 656 T_(s) (° C.) a (10⁻⁷/° C.) 74.2 74.281.3 81.2 73.5 78.1 88.9 73 r (gm/cc) 2.424 2.485 2.451 2.435 2.5852.428 2.889 2.933 T₂₀₀ (° C.) 1637 1669 1586 1611 1613 1615 1491 13471406 T_(liq) (° C.) 800 940 980 945 1090 960 1000 1005 η_(liq) (kp)427,000 4527 301 1829 107 665 59 156

TABLE 8 Example 50 51 52 53 54 55 56 Composition (mol %) K₂O 11.79 11.9112.04 11.25 8.96 10.9 10.79 MgO 6.03 4.95 3.87 6.3 7.06 6.11 6.05 CaO5.64 5.39 5.13 5.53 6.21 5.36 5.31 SrO 1.69 1.32 0.94 1.8 2.01 1.74 1.72BaO 0 0 0 0 0 0 0 B₂O₃ 1.66 1.88 2.11 5.55 1.45 1.44 2.44 Al₂O₃ 3.053.32 3.6 2.79 2.72 3.22 3.19 SiO₂ 70.04 71.13 72.21 66.68 71.5 71.1370.4 SnO₂ 0.1 0.1 0.1 0.1 0.1 0.1 0.1 Composition (wt %) K₂O 17.06 17.1817.3 16.27 13.22 15.84 15.66 MgO 3.73 3.05 2.38 3.9 4.45 3.8 3.76 CaO4.86 4.63 4.39 4.76 5.45 4.63 4.59 SrO 2.69 2.1 1.49 2.86 3.26 2.78 2.74BaO 0 0 0 0 0 0 0 B₂O₃ 1.78 2.01 2.25 5.93 1.58 1.55 2.62 Al₂O₃ 4.785.18 5.6 4.36 4.34 5.06 5.01 SiO₂ 64.66 65.43 66.18 61.49 67.27 65.9165.19 SnO₂ 0.23 0.23 0.23 0.23 0.24 0.23 0.23 T_(str) (° C.) 596 597 601612 627 624 622 T_(ann) (° C.) 645 646 651 660 679 671 674 T_(s) (° C.)a (10⁻⁷/° C.) 83.3 82.6 82.5 71.9 72.4 78.3 74.9 r (gm/cc) 2.486 2.4732.458 2.48 2.483 2.477 2.473 T₂₀₀ (° C.) 1543 1560 1608 1557 1560 15761613 T_(liq) (° C.) 995 985 950 1080 1095 1020 1055 η_(liq) (kp) 480 6882361 76 96 553 329

TABLE 9 Example 57 58 59 60 61 62 63 Composition (mol %) K₂O 10.15 9.953.35 3.75 4.1 3.64 10.1 MgO 5.69 5.58 5.39 6.04 6.59 5.87 5.66 CaO 4.994.89 5.57 6.24 6.81 6.06 4.96 SrO 1.62 1.59 2.3 2.58 2.81 2.51 1.61 BaO0 2 0 0 0 0 0 B₂O₃ 1.34 1.31 5.69 5.69 5.69 5.53 1.33 Al₂O₃ 3 2.94 11.2610.46 9.78 10.16 3.5 SiO₂ 73.11 71.64 66.37 65.17 64.15 66.17 72.74 SnO₂0.1 0.1 0.07 0.07 0.07 0.07 0.1 total 100 100 100 100 100 100.01 100Composition (wt %) K₂O 14.82 14.14 4.75 5.34 5.86 5.2 14.7 MgO 3.55 3.393.27 3.68 4.03 3.58 3.52 CaO 4.34 4.13 4.7 5.29 5.79 5.15 4.3 SrO 2.612.49 3.59 4.04 4.41 3.94 2.58 BaO 0 4.62 0 0 0 0 0 B₂O₃ 1.45 1.38 5.975.99 6.01 5.84 1.43 Al₂O₃ 4.74 4.52 17.3 16.12 15.15 15.7 5.51 SiO₂68.09 64.94 60.09 59.2 58.44 60.26 67.54 SnO₂ 0.23 0.23 0.16 0.16 0.160.16 0.23 total 99.83 99.84 99.83 99.82 99.85 99.83 99.81 Tstr ((° C.)609 598 660 645 632 645 621 Tann (° C.) 658 645 714 694 678 694 671 Ts(° C.) a (10−7/° C.) 76.2 79.7 46.7 52.8 55.5 50 75 r (gm/cc) 2.4622.544 2.463 2.504 2.517 2.474 2.46 T200 (° C.) 1605 1569 1609 1597 15551600 1613 Tliq (° C.) 1000 950 1110 1080 1080 1065 1025 nliq (kp) 9672257 287 432 228 695 682

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the present inventionwithout departing from the spirit or scope of the invention. Thus, it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1. A glass comprising, in weight percent: 35 to 75 percent SiO₂; 0 to 15percent Al₂O₃; 0 to 20 percent B₂O₃; 3 to 30 percent K₂O; 0 to 15percent MgO; 0 to 10 percent CaO; 0 to 12 percent SrO; 0 to 40 percentBaO; and 0 to 1 percent SnO₂, wherein the glass is substantially free ofNa₂O.
 2. The glass according to claim 1, comprising: greater than 0percent B₂O₃.
 3. The glass according to claim 1, wherein the glass issubstantially free of B₂O₃.
 4. The glass according to claim 1,comprising: at least 45 percent SiO₂.
 5. The glass according to claim 1,comprising: greater than 0 percent MgO, CaO, SrO, or combinationsthereof.
 6. The glass according to claim 1 comprising, in weightpercent: 35 to 75 percent SiO₂; greater than 0 to 15 percent Al₂O₃;greater than 0 to 20 percent B₂O₃; 3 to 30 percent K₂O; greater than 0to 15 percent MgO; greater than 0 to 10 percent CaO; greater than 0 to12 percent SrO; greater than 0 to 40 percent BaO; and greater than 0 to1 percent SnO₂, wherein the glass is substantially free of Na₂O.
 7. Theglass according to claim 1, comprising: 39 to 75 percent SiO₂; 2 to 13percent Al₂O₃; 1 to 11 percent B₂O₃; 3 to 30 percent K₂O; 0 to 7 percentMgO; 0 to 10 percent CaO; 0 to 12 percent SrO; 0 to 40 percent BaO; and0 to 1 percent SnO₂, wherein the glass is substantially free of Na₂O. 8.The glass according to claim 1, comprising: 50 to 70 percent SiO₂; 2 to13 percent Al₂O₃; 1 to 11 percent B₂O₃; 3 to 30 percent K₂O; 0 to 7percent MgO; 0 to 7 percent CaO; 0 to 5 percent SrO; 1 to 40 percentBaO; and 0 to 0.3 percent SnO₂, wherein the glass is substantially freeof Na₂O.
 9. The glass according to claim 1, wherein the glass is in theform of a sheet.
 10. The glass according to claim 9, wherein the sheethas a thickness in the range of from 0.5 mm to 3.0 mm.
 11. Aphotovoltaic device comprising the glass according to claim
 1. 12. Thephotovoltaic device according to claim 11, comprising a functional layercomprising copper indium gallium diselenide or cadmium tellurideadjacent to the substrate or superstrate.
 13. The photovoltaic deviceaccording to claim 12, further comprising a barrier layer disposedbetween the superstrate or substrate and the functional layer.
 14. Theglass according to claim 1, having a strain point of 540° C. or greater.15. The glass according to claim 1, having a coefficient of thermalexpansion of 50×10⁻⁷ or greater.
 16. The glass according to claim 1,having a coefficient of thermal expansion in the range of from 50×10⁻⁷to 90×10⁻⁷.
 17. The glass according to claim 1, having a strain point of560° C. or greater and a coefficient of thermal expansion of 50×10⁻⁷ orgreater.
 18. The glass according to claim 1, having a liquidus viscosityof 50,000 poise or greater.
 19. The glass according to claim 1, having aT₂₀₀ less than 1580° C. and a liquidus viscosity of 400,000 poise orgreater.
 20. The glass according to claim 1, wherein the glass is fusionformable and has a strain point of 540° C. or greater, a coefficient ofthermal expansion of 50×10⁻⁷ or greater, T₂₀₀ less than 1630° C., andhaving a liquidus viscosity of 150,000 poise or greater.
 21. A glassconsisting essentially of, in weight percent: 35 to 75 percent SiO₂; 0to 15 percent Al₂O₃; 0 to 20 percent B₂O₃; 3 to 30 percent K₂O; 0 to 15percent MgO; 0 to 10 percent CaO; 0 to 12 percent SrO; 0 to 40 percentBaO; and 0 to 1 percent SnO₂, wherein the glass is substantially free ofNa₂O.
 22. A glass comprising, in weight percent: 45 to 75 percent SiO₂;3 to 15 percent Al₂O₃; 0 to 20 percent B₂O₃; 14 to 25 percent K₂O; 0 to15 percent MgO; 0 to 10 percent CaO; 0 to 12 percent SrO; 0 to 40percent BaO; and 0 to 1 percent SnO₂, wherein the glass is substantiallyfree of Na₂O and wherein the glass is fusion formable and has a strainpoint of 540° C. or greater, a coefficient of thermal expansion of50×10⁻⁷ or greater, T₂₀₀ less than 1630° C., and having a liquidusviscosity of 150,000 poise or greater.
 23. A glass comprising, in weightpercent: 45 to 75 percent SiO₂; 3 to 15 percent Al₂O₃; 0 to 20 percentB₂O₃; 3 to 30 percent K₂O; 0 to 15 percent MgO; 0 to 10 percent CaO; 0to 12 percent SrO; 0 to 40 percent BaO; and 0 to 1 percent SnO₂, whereinthe glass is substantially free of Na₂O and wherein the glass is fusionformable and has a strain point of 540° C. or greater, a coefficient ofthermal expansion of 50×10⁻⁷ or greater, T₂₀₀ less than 1630° C., andhaving a liquidus viscosity of 150,000 poise or greater.